Ch. 6 The Muscular System NOTES The Muscular System Muscles are responsible for all types of body movement Three basic muscle types are found in the body o Skeletal muscle o Cardiac muscle o Smooth muscle Characteristics of Muscles o Muscle cells are elongated (muscle cell = muscle fiber) o Contraction of muscles is due to the movement of microfilaments o All muscles share some terminology Prefix myo refers to muscle Prefix mys refers to muscle Prefix sarco refers to flesh Skeletal Muscle Characteristics: o Most are attached by tendons to bones o Cells are multinucleate o Striated have visible banding o Voluntary subject to conscious control o Cells are surrounded and bundled by connective tissue o Connective tissue wrappings of skeletal muscle: Endomysium around single muscle fiber Perimysium around a fascicle (bundle) of fibers Epimysium covers the entire skeletal muscle Fascia on the outside of the epimysium o Skeletal muscle attachments: Epimysium blends into a connective tissue attachment Tendon cord-like structure Aponeuroses sheet-like structure Sites of muscle attachment Bones Cartilages Connective tissue coverings Page 1
Smooth Muscle Characteristics: o Has no striations o Spindle-shaped cells o Single nucleus o Involuntary no conscious control o Found mainly in the walls of hollow organs Cardiac Muscle Characteristics: o Has striations o Usually has a single nucleus o Joined to another muscle cell at an intercalated disc o Involuntary o Found only in the heart Function of Muscles: Produce movement Maintain posture Stabilize joints Generate heat Microscopic Anatomy of Skeletal Muscle: Cells are multinucleate Nuclei are just beneath the sarcolemma o Sarcolemma specialized plasma membrane o Sarcoplasmic reticulum specialized smooth endoplasmic reticulum o Myofibril -- bundles of myofilaments Myofibrils are aligned to give distinct bands I band = light band A band = dark band Sarcomere -- Contractile unit of a muscle fiber Organization of the sarcomere: Thick filaments = myosin filaments o Composed of the protein myosin Page 2
o Has ATPase enzymes Thin filaments = actin filaments o Composed of the protein actin Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat At rest, there is a bare zone that lacks actin filaments Sarcoplasmic reticulum (SR) for storage of calcium Properties of Skeletal Muscle Activity: Irritability ability to receive and respond to a stimulus Contractility ability to shorten when an adequate stimulus is received Nerve Stimulus to muscles: o Skeletal muscles must be stimulated by a nerve to contract Motor unit -- one neuron Muscle cells stimulated by that neuron o Neuromuscular junctions association site of nerve and muscle o Synaptic cleft gap between nerve and muscle Nerve and muscle do not make contact Area between nerve and muscle is filled with interstitial fluid o Transmission of nerve impulse to muscle: The neurotransmitter for skeletal muscle is acetylcholine Neurotransmitter attaches to receptors on the sarcolemma Sarcolemma becomes permeable to sodium (Na + ) Sodium rushing into the cell generates an action potential Once started, muscle contraction cannot be stopped Page 3
The Sliding Filament Theory of Muscle Contraction: o Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin filament o Myosin heads then bind to the next site of the thin filament o This continued action causes a sliding of the myosin along the actin o The result is that the muscle is shortened (contracted) Contraction of a skeletal muscle: o Muscle fiber contraction is all or none o Within a skeletal muscle, not all fibers may be stimulated during the same interval o Different combinations of muscle fiber contractions may give differing responses o Graded responses different degrees of skeletal muscle shortening Types of Graded Responses Twitch -- single, brief contraction (not a normal muscle function) Tetanus (summing of contractions) one contraction is immediately followed by another o The muscle does not completely return to a resting state o The effects are added Unfused (incomplete) tetanus some relaxation occurs between contractions o The results are summed Fused (complete) tetanus no evidence of relaxation before the following contractions o The result is a sustained muscle contraction Page 4
Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy Energy for Muscle Contraction o Initially, muscles used stored ATP for energy Bonds of ATP are broken to release energy Only 4-6 seconds worth of ATP is stored by muscles o After this initial time, other pathways must be utilized to produce ATP o o Direct phosphorylation Muscle cells contain creatine phosphate (CP) CP is a high-energy molecule After ATP is depleted, ADP is left CP transfers energy to ADP, to regenerate ATP CP supplies are exhausted in about 20 seconds Aerobic Respiration Series of metabolic pathways that occur in the mitochondria Glucose is broken down to carbon dioxide and water, releasing energy This is a slower reaction that requires continuous oxygen Page 5
o o o Anaerobic glycolysis Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce some ATP Pyruvic acid is converted to lactic acid Anaerobic glycolysis (continued) This reaction is not as efficient, but is fast Huge amounts of glucose are needed Lactic acid produces muscle fatigue Muscle Fatigue & Oxygen Debt When a muscle is fatigued, it is unable to contract The common reason for muscle fatigue is oxygen debt Oxygen must be repaid to tissue to remove oxygen debt Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less Types of Muscle Contractions Isotonic contractions o Myofilaments are able to slide past each other during contractions o The muscle shortens Isometric contractions o Tension in the muscles increases o The muscle is unable to shorten Muscle Tone o Some fibers are contracted even in a relaxed muscle o Different fibers contract at different times to provide muscle tone o The process of stimulating various fibers is under involuntary control Muscles & Body Movements Movement is attained due to a muscle moving an attached bone Muscles are attached to at least two points o Origin attachment to a moveable bone o Insertion attachment to an immovable bone Page 6
Effects of Exercise on Muscle Results of increased muscle use o Increase in muscle size o Increase in muscle strength o Increase in muscle efficiency o Muscle becomes more fatigue resistant Types of Ordinary Body Movements Flexion brings two bones closer together Extension moving two bones farther apart o hyperextension an extension that is greater than 180º Rotation movement of a bone around its longitudinal axis Abduction moving a limb away from the midline o example: fanning your fingers or toes apart Circumduction the proximal end of the limb is stationary, and its distal end moves in a circle Page 7
Special Movements: Dorsifelxion flexing your foot Plantar flexion pointing your foot Inversion turn the sole of your foot medially Eversion turn the sole of your foot laterally Supination palms face anteriorly so the radius and ulna are parallel Pronation palms face posteriorly so that the two bones form an X Opposition the saddle joint in your palm allows your thumb to move independently Types of Muscles: Prime mover muscle with the major responsibility for a certain movement Antagonist muscle that opposes or reverses a prime mover Synergist muscle that aids a prime mover in a movement and helps prevent rotation Fixator stabilizes the origin of a prime mover Page 8
Naming of Skeletal Muscles: Direction of muscle fibers o Example: rectus (straight) Relative size of the muscle o Example: maximus (largest) Location of the muscle o Example: many muscles are named for bones (e.g., temporalis) Number of origins o Example: triceps (three heads) Location of the muscles origin and insertion o Example: sterno (on the sternum) Shape of the muscle o Example: deltoid (triangular) Action of the muscle o Example: flexor and extensor (flexes or extends a bone) Page 9
Superficial muscles: Anterior: Posterior: Page 10